Looking for the title of a movie in which people disappear

I saw a movie a couple a months ago. The movie is about a teenage girl that lives in world in which people has/are disappearing. People just suddenly turn into dust. Also I think some people show up then and again as "ghosts" replaying some short part of their lives.In one scene she is in the classroom when a classmate reappear like this. The girl teams up with a nerd I think and solves the problem?